An exemplary conventional high capacity magnetic storage system typically uses magnetoresistive read (MR) sensors to read data from a surface of the magnetic disk. An MR sensor comprises an MR sensing layer. Data stored in the form of a magnetic field emanating from the magnetic disk changes the resistance of the MR sensing layer. This change in resistance allows the MR sensor to detect the magnetic field on the magnetic disk. The resistance of the MR sensor changes as a function of the magnitude and direction of the magnetic flux from the magnetic disk.
A giant magnetoresistive (GMR) sensor is a type of MR sensor that comprises a GMR stack. The GMR stack includes a plurality of magnetic layers that are separated by a non-magnetic layer. The magnetization of one of the magnetic layers (the pinned layer) is pinned by exchange coupling with an antiferromagnetic layer.
Another magnetic layer (the free layer) is not pinned; the magnetic moments in this layer are free to rotate in response to the field from the magnetic disk. The electrical resistance of GMR sensor depends on the relative alignment of the magnetic moments in the free layer and the pinned layer. The magnetic field from the magnetic disk induces a change in the direction of magnetization in the free layer, thus changing the resistance of the GMR sensor.
The change in resistance of the GMR sensor can be measured by applying a current to the GMR sensor. The GMR sensor comprises conductive lead structures that connect to the GMR stack, providing means for applying current to the GMR stack. The change in resistance of a GMR sensor is typically determined by applying a constant current and measuring voltage variations caused by the change in resistance. Conventional GMR sensors are biased by a permanent magnetic known as a hard bias (HB). The hard bias provides a preferred direction or “off” resistance for the free layer in the GMR stack.
Although this technology has proven to be useful, it would be desirable to present additional improvements. A large hard bias layer is desired for better thermal and magnetic stability of the GMR sensor. However, as the hard bias layer increases in size, the “on” resistance of the free layer decreases, providing a lower amplitude voltage (or output signal) when reading the magnetic disk.
As the areal density of magnetic disks increases, magnetic read widths decrease. At very low magnetic read widths (below approximately 0.15 micron), the two sides of the permanent magnetic material in the hard bias are very close. Consequently, the hard bias field in the GMR sensor could cause the output signal to be below an acceptable level.
To maintain the amplitude of the output signal from the GMR sensor, the strength of the magnetic field applied by the permanent magnetic material in the hard bias is decreased. A hard bias at these small thicknesses is however susceptible to degradation.
At very low track widths, the hard bias becomes very thin and the junctions of the hard bias closest to the GMR stack become unstable. At high temperature, the junctions of the hard bias become demagnetized, allowing an increase in side reading. The read width of the MR sensor essentially becomes wider. In addition, magnetic instability of the device response is introduced by the weakening of the hard bias.
Furthermore, the top shield region that overlies the GMR stack should be as flat as possible to minimize side reading by the GMR sensor. At very low magnetic read widths (i.e., below approximately 0.15 micron), the conventional topography of the conductive leads does not allow this top shield region to be flat across substantially the entire width of the GMR stack, thus increasing undesirable side reading.
Therefore, what is needed is an MR read sensor with improved stability, increased amplitude of the output signal, and reduced side reading. The need for such an MR read sensor has heretofore remained unsatisfied.